Many industries require the use of heat exchangers to regulate the temperature of high purity and/or corrosive fluids. For example, microchip fabrication within the semiconductor industry requires heating and temperature regulation of the etching and/or cleaning fluids used to etch and/or clean silicon wafers and microcircuit lines. Because both the process temperatures and the heat capacities of the etching/cleaning fluids are relatively high, a rather large amount of heat is required to raise and maintain the temperature of the etching/cleaning fluid.
Due to the corrosive nature of the typical etching/cleaning fluids used in the semiconductor industry, common materials traditionally utilized in the fabrication of heat exchangers such as metals are not chemically compatible, and therefore, are unacceptable. While metals are extremely good thermal conductors, they are chemically attacked when exposed to these corrosive fluids. As a result, the fluid becomes contaminated and can no longer be used as an etching/cleaning agent.
In order to solve this limitation, a chemically inert material such as Teflon™ is used to either carry the fluid or to protect the resistive element from being corroded, as in the case of an immersion type heater. Although chemically inert, Teflon™ is a very poor conductor, and therefore, the thermal transfer between the heat source and the fluid is limited. There are currently two configurations of heat exchangers that utilize Teflon™ to maintain both chemical compatibility and purity. The first, and most common configuration, is referred to as an immersion heater. The immersion type heaters utilize large vessels with immersed heating coils that are encased by a chemically inert material such as Teflon™. Because Teflon™ is a relatively poor conductor, a very thin layer of Teflon™ is used in order to minimize the thermal resistance between the heating element and the fluid being heated. Also, in order to increase the thermal transfer to the fluid, it is necessary to maximize the surface area between the heating element and the fluid. Therefore, large lengths of the heating element are packed in a coil arrangement inside the vessel. These coils result in “dead” zones where particles reside and shed over time. This makes the described arrangement less desirable for high purity applications. This is unacceptable because, due to stringent process requirements, etching/cleaning fluids must be free of foreign particles in order to avoid the contamination and destruction of microcircuits formed in the silicon wafers.
Another problem associated with immersion type heaters is related to the geometry of such coils. As the fluid flows across these coils, stagnant regions are formed. These are regions where no fluid flow is present and/or regions where no fluid ever comes in contact with the heating element (“micro bubbles”). Stagnant regions can lead to “hot spots” which are areas where high temperature gradients exist. High temperature gradients can often times degrade the chemical (e.g., lead to premature chemical aging). The combination of hot spots and the micro-bubbles greatly reduce both the efficiency of the heat exchanger and the heating element life, and can also lead to chemical degradation. Another common problem associated with immersion type heat exchangers is that the thin layer of Teflon™ burns immediately when the heating elements become exposed to air. This is a common mode of failure that significantly increases maintenance and parts replacement costs.
Another configuration of heat exchanger currently in use is illustrated in the example of U.S. Pat. No. 5,899,077 issued to Wright, et al., which describes a type of heat exchanger where inert tubing, such as Teflon™ tubing, is sandwiched between thermally conductive rectangular plates. This heat exchanger has been designed to control the temperature of fluids within the room-temperature range. In this configuration, the temperature of the thermally conductive material is controlled via thermoelectric modules. Although thermoelectric modules are useful devices that can cool and heat the conductive material, they are limited to low wattage applications. Hence, this heat exchanger device would not be suitable for heating the common semiconductor etching and cleaning fluids.
Another problem associated with the design described in the prior art listed above, is that it is very difficult to form tight bends in known inert tubing materials. This creates several problems when designing and manufacturing heat exchangers, wherein tubing typically includes multiple bends. First, known inert tubing is easily kinked, and cannot therefore be bent into small diameter bends. Rather, such tubing requires a large bend radius and is, therefore, often bent outside of the heat exchanger, thereby reducing the heating efficiency of the heat exchanger and increasing its size. Further, as the wall thickness of the tubing decreases, the required bend radius increases. Alternatively, if the tubing is entirely retained within the heat exchanger, a complex curved channel with large bend radii must be machined into the conductive plates. In either situation, because of the large bend radii of the plastic tubing, less tubing can be used per unit surface area of the heat exchanger, thereby reducing the thermal efficiency of the heat exchanger and dramatically increasing its size.
In order to compensate for the limited surface area caused by the limited number of bends and limited overall quantity of tubing that can be sandwiched between the rectangular conductive plates, coiled inserts are sometimes placed within the tube. While the turbulence caused by the inserts facilitates increased thermal transfer between the heat exchanger and the fluid, the inserts also cause dead zones within the fluid flow, increasing the potential for particle build-up and contamination of the etching/cleaning fluid. In addition to the coiled inserts, thinner walled inert tubing is often used in order to increase the thermal conduction between the plates and the fluid. While reducing the tubing wall thickness enhances the heat transfer between the conductive plate and fluid, it dramatically reduces the pressure rating of the inert tubing and dramatically increases its bend radius. This severely limits the temperature and pressure ranges within which the heat exchanger can operate, making such solutions unsuitable for many heating applications.
What is needed, therefore, is a heat exchanger design that allows for increased inert tubing surface area while remaining compact. It would also be useful to have a heat exchanger design where no stagnant areas and dead zones exist and/or a heat exchanger that can withstand high pressures at elevated temperatures.